This invention relates generally to optical switches and, more specifically, to opto-mechanical optical switches.
Robust, high performance optical switches are necessary for route diversity, for dense wavelength division multiplexing (DWDM) switchable add/drop multiplexing, and for optical cross-connects [OXC]. Although solid state approaches such as thermo-optic switches appear attractive, they suffer in insertion loss and crosstalk performance when compared with opto-mechanical solutions. For systems requiring intermediate switching speeds for reconfiguration, rerouting, and route protection, opto-mechanical fiber optic switches can provide the right mix of cost and reliability. For OXC applications, opto-mechanical switches can also offer scalability to a practical level.
One approach for opto-mechanical fiber optic switches is a centro-symmetric reflective [CSR] imaging and switching device. Unlike other opto-mechanical switches which use a combination of GRIN lenses and interposing prisms or mirrors to effect switching, this device uses a single concave spherical mirror to image the light from an array of fibers. Developed in 1978 the CSR approach provides one-to-one imagery and is achromatic and diffraction limited, providing the object and image are not too far from the axis of the reflector. As shown in FIG. 1, a rotating single concave spherical mirror 10 effects rotating switching among an array of fibers 11. This approach has been used in generations of multi-mode optical bypass switches. This approach reduces the complexity of the optical system and allows all of the optical paths to be controlled with common optical elements, whilexe2x80x94providing the point-to-point imaging necessary for low insertion loss.
Although this approach is effective for multi-mode switching, it is generally considered unsuitable for single-mode switching. More specifically, in single-mode switching, the mechanical slop in the pivot assembly causes an unacceptable amount of loss fluctuation when the mirror is the moving part. Therefore, there is a need for a fiber switching device for single-mode applications which offers the benefits of opto-mechanical switching but does not suffer from the imprecision traditionally associated with opto-mechanical switching devices.
The present invention provides a highly-precise, opto-mechanical approach for fiber switching that avoids mechanical play in the pivot assembly of the mirror by keeping the mirror stationary and instead moving one or more refractive plates interposed between the mirror and the array of fibers to shift the light beams between the fibers and the mirror and thereby effect the desired switching among the fibers. This approach reduces the number of mechanical degrees of freedom which negatively affect the optical image positions, thus easing manufacturing and alignment tolerances. Additionally, this approach enables the sensitivity of the system to the rotation of the refractive plane to be tuned by varying the dimensions of the refractive plate. Another advantage of using plane plates is that the displacement of the image is independent of the x,y,z position of each plate, thus, only the tilt of the plates matter. This is a significant advantage when the switch is thermally cycled.
The opto-mechanical switching approach of the present invention is particularly advantageous for more complex applications than protection switching, such as OXC, where it is necessary to scale simple 1xc3x972 switches to larger configurations. While solid-state switches and some opto-mechanical switches are generally scaled by creating tree structures of 1xc3x972 switches, this typically means switch losses are additive. Compared to switches based on an individual lens for each fiber, the approach of the present invention can be scaled to 1xc3x97N with much greater economy of both cost and insertion loss by simply increasing the number of refractive plates.
Accordingly, one aspect of the invention is an opto-mechanical switching devices that effects switching through the movement of refractive plates interposed between the mirror and the fiber array. In a preferred embodiment, the device comprises: (a) an input optical fiber; (b) a plurality of output optical fibers; (c) a rigid concave spherical mirror; and (d) at least one planar refractive plate located between the input fiber and the mirror and shiftable between a first position and a second position wherein the refractive plate has a thickness dimension in the direction through which a light beam from the input optical fiber passes and an index of refraction that are selected to transmit the light beam to and from the mirror through the refractive plate to different output optical fibers when the refractive plate is switched between the first position and the second position.